The central dogma of biology states that DNA is transcribed into RNA, which is then translated into protein. However, this process is complex, and there are a number of intermediates that facilitate the transfer of genetic information from one molecule to another. In particular, there are three types of RNA that serve as mediators in this process: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). However, these molecules are modified, or editied, to enhance their ability to sucessfully transfer genetic information into protein complexes that afford a cell or organism viability, variation, and optimal ability to function.

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RNA editing is a necessary step in the gene expression of many organisms. It can be either a co-transcriptional processing event or post-transcriptional alteration of a transcript, resulting in a mature transcript sequence that is altered from the genomic sequence. Other co- or post-transcriptional changes such as 5' capping, 3' polyadenylation, and RNA splicing also modify the RNA transcript, but differ from RNA editing as they do not change the sequence of the mature transcript. RNA editing is a widespread phenomenon that occurs in the RNAs of all eukaryotes. It has been observed in mRNA, tRNA, and rRNA, and the effect of editing has been studied in mitochondria, choloroplasts, and nuclear encoded RNAs, for example.

There are several types of RNA editing. RNA editing occurs in all types of RNAs, most commonly in mRNAs, but also in structural RNAs such as tRNAs and rRNAs. mRNA editing can be grouped into two basic classes: insertion/deletion editing and substitution editing, which includes C to U Editing and A to I Editing. Though most of the editing is observed in mRNA, even structural RNA such as rRNA and tRNA are also edited. Click on one of the above links to begin learning about RNA editing.

Often the genetic information in the DNA is not complete and may not give a functional protein, structural RNA, or other product. Thus, RNA editing is necessary for expression of genes in many organisms. Editing corrects frameshifts, creats translation initiation codons, and converts the transcripts of unrecognizable cryptogenes into translatable mRNAs (Simpson et al 1997). Editing helps mRNA molecules to leave the nucleus and function correctly in the cell, and in many cases, editing is essential for the survival of the organism, as editing of RNA is necessary to produce altered, but functional products. Editing is particularly important in mitochondria and plastids where conservation of sequence with their homologues is observed only after editing. tRNA Editing leads to the proper folding of tRNAs. This is form of regulatory control which results in the translation of a transcript by modifying an previously synthesized transcript. Another advantage of RNA editing is it can result in two kinds of transcripts from the same coding region (Brennicke et al 1999). The complexity of the proteome in higher eukaryotes is an order of magnitude higher than that predicted by the genome sequence. Protein diversity can be expanded by altering the sequence of genes, regulating gene expression and altering transcript sequences. RNA editing is one of the processes, along with genetic recombination, alternative splicing of pre-mRNA, alternative 3'-end formation, and others events that leads to greater complexity of the proteome as compared to the genome (Wedekind et al 2003).

Editing is an expensive process for the cell, since editing is not necessary if the sequence of the mature RNA is already encoded in the genome. Therefore, the biological significance of editing should not be overlooked. RNA editing is required for expression of genes encoding proteins or structural tRNAs in many organisms, creates protein diversity, and affects the physiological functions of proteins. Although editing may serve as a kind of post-transcriptional regulatory control, diverse but independent mechanisms of RNA editing are seen in different organisms and also in different types of RNAs. Thus, RNA editing mechanisms seem to have evolved to compensate for the alteration of gene sequences by mutations or to increase variation (Brennicke et al 1999), actions that could potentially stimulate evolution.

mRNA Insertion and Deletion (U): Minu

mRNA Substitution (A to I): Alex

mRNA Substitution (C to U): Sephorah

tRNA Editing: Haritha

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